1. Field of the Invention
The present invention relates to a novel iridium complex that is capable of converting a triplet excited state into luminescence. In addition, the present invention relates to a light-emitting element, a light-emitting device, an electronic device, and a lighting device each using the iridium complex.
2. Description of the Related Art
In recent years, a light-emitting element which uses an organic or inorganic compound having a light-emitting property as a light-emitting material has been actively developed. In particular, a light-emitting element called an EL (electroluminescence) element has attracted attention as a next-generation flat panel display element because it has a simple structure in which a light-emitting layer containing a light-emitting material is provided between electrodes, and characteristics such as feasibility of being thin, lightweight, and highly responsive to input signals, and able to be driven with direct current at a low voltage. In addition, a display using such a light-emitting element has a feature that it is excellent in image quality such as contrast, and has a wide viewing angle. Further, since such a light-emitting element can perform planar light emission, the light-emitting element is considered to be applicable to a light source such as a backlight of a liquid crystal display and lighting.
In an EL element in which an organic compound having a light-emitting property is used as a light-emitting substance, by applying a voltage with a light-emitting layer interposed between electrodes, electrons and holes injected from the electrodes recombine to put the light-emitting substance into an excited state, and light is emitted when the light-emitting substance returns to a ground state from the excited state. The excited states of the light-emitting substance may be a singlet excited state (S*) and a triplet excited state (T*), and the statistical generation ratio thereof in the element is considered to be S*:T*=1:3.
In general, the ground state of an organic compound having a light-emitting property is a singlet state. Therefore, in light emission from a singlet excited state (S*), which is called fluorescence, electron transition occurs between the same spin multiplicities. In contrast, in light emission from a triplet excited state (T*), which is called phosphorescence, electron transition occurs between different spin multiplicities.
In an organic compound having a light-emitting property and emitting fluorescence (hereinafter referred to as fluorescent compound), in general, phosphorescence is not observed at room temperature, and only fluorescence is observed. Accordingly, the internal quantum efficiency in a light-emitting element using a fluorescent compound is presumed to have a theoretical limit of 25% based on S*:T*=1:3 unless a special measure (e.g., utilization of thermally activated delayed fluorescence) is taken.
In contrast, internal quantum efficiency in a light-emitting element using an organic compound having a light-emitting property and emitting phosphorescence (hereinafter referred to as phosphorescent compound) can be 100% in theory when light emission led from intersystem crossing from S* to T* is taken into account. For this reason, the light-emitting element using a phosphorescent compound has been actively developed in order to achieve a highly efficient light-emitting element.
As a phosphorescent compound, an organometallic complex having iridium or the like as a central metal has attracted attention. Development has enabled phosphorescent compounds to emit light with various wavelengths from red to blue; however, for phosphorescence, i.e., light emission from a triplet excited state which is at a lower energy level than a singlet excited state, an organometallic complex having a wider energy gap is necessary for obtaining green to blue light emission with short wavelengths. Such substances are difficult to develop and the number thereof is still limited.
Patent Document 1 discloses an iridium complex in which an imidazole derivative is a ligand.